Not all vitamin C is the same: L-ascorbic acid, THD ascorbate and what the difference means

Two vitamin C serums sit side by side on the shelf. One is a thin clear liquid. The other looks like a golden oil. Both say vitamin C on the front, because vitamin C on a label is a family of ingredients rather than one ingredient.

The two you will see most often are L-ascorbic acid, which is the pure vitamin, and tetrahexyldecyl ascorbate, usually shortened to THD ascorbate. There is no head to head human trial comparing them at matched strengths, so nobody can honestly tell you which is better. What we can do is set out what each one has behind it, and where that evidence runs out.

The names on the ingredients list

They are not interchangeable. They behave differently in the bottle and on the skin.

• L-ascorbic acid. The pure vitamin. Water-soluble, needs a low pH to get into skin, and the form with the most human research behind it.
• Tetrahexyldecyl ascorbate, or THD ascorbate. Oil-soluble. Described in the research as a precursor, meaning the skin has to convert it. It sits happily in a normal-pH oil or cream base, and it has far less direct research behind it.
• Magnesium ascorbyl phosphate and ascorbyl palmitate. Two older modified forms, and not the same as each other either.

Several modified forms of vitamin C were developed to try to improve on the stability and skin absorption of plain L-ascorbic acid, and a 2022 review in a cosmetic dermatology journal set out to compare them (Enescu et al., 2022). The thing worth taking from that is simply that they are separate ingredients. A result shown for one form does not carry across to the others, which is the assumption most vitamin C marketing quietly relies on.

What the human research on L-ascorbic acid shows

Two trials come up again and again, and both are worth reading properly rather than taking on trust.

The longer one was a six-month double-blind randomised trial. A 5 per cent vitamin C cream was applied daily to the low neck and arms of female volunteers with sun-exposed skin, against the same cream without the vitamin C. A dermatologist assessment and the volunteers' own assessment both found a significant improvement in the overall score on the treated side. Silicone replicas of the skin surface showed an increase in the density of skin microrelief and a decrease in deep furrows. It was well tolerated (Humbert et al., 2003).

The shorter one was a three-month randomised, double-blind, vehicle-controlled split-face study in 19 people aged 36 to 72, with Fitzpatrick skin types I to III (Traikovich, 1999). Worth knowing: both the active serum and the matched vehicle it was tested against were a single manufacturer's own products.

Now the size of that evidence base. These are two small, short studies. The six-month one does not report how many volunteers took part, and all of them were women, on the neck and arms rather than the face. The one that recorded skin type included only the three lightest of the six Fitzpatrick types, so we cannot tell you how it reads across darker skin. Both measured the look and the surface texture of skin, not the health of it. That is still more direct human evidence than any other form of vitamin C has, which tells you how thin the whole field is rather than how strong L-ascorbic acid is.

The catch is chemistry

An absorption study applied L-ascorbic acid and several modified forms to pig skin and measured what actually got in. L-ascorbic acid had to be formulated below pH 3.5 to enter the skin at all. The best concentration for absorption was 20 per cent. Skin levels stopped rising after three daily applications. The modified forms tested, which were magnesium ascorbyl phosphate, ascorbyl-6-palmitate and dehydroascorbic acid, did not raise skin levels of L-ascorbic acid (Pinnell et al., 2001).

Two caveats before anyone builds a rule on that. It is pig skin, not human faces. And it did not test THD ascorbate, so it says nothing at all about that form.

With that said, it is the reason we are sceptical of very high percentages. If 20 per cent was the best concentration for absorption in that model, a 30 per cent serum is not obviously doing more work than a 15 or 20 per cent one, and we have not seen published evidence that it does.

The low pH is also the practical downside. An L-ascorbic acid serum has to be acidic to get in, and plenty of people tell us in store that an acidic serum tingles, stings or leaves their skin feeling tight, particularly if their skin is dry or reacts easily. That is not a finding from the study above, it is what we hear across the counter.

Why your serum has gone brown

L-ascorbic acid in water is genuinely fragile. Laboratory testing of ascorbic acid, ascorbyl palmitate and magnesium ascorbyl phosphate found plain ascorbic acid the least stable of the three, in solution and in finished formulations. The two modified forms held up better, but not equally: ascorbyl palmitate still broke down, while magnesium ascorbyl phosphate was stable (Austria et al., 1997). THD ascorbate was not in that study. The base formula matters as much as the choice of ingredient.

So if your serum has gone from clear or pale straw to deep yellow, amber or brown, it has oxidised. It is not dangerous, but it is spent. Bin it rather than pushing through the last third, because there is no way to bring it back.

Colour is a rough signal rather than a lab test. A serum can lose potency before it visibly browns, and some formulas darken a little because of other ingredients such as ferulic acid, so a faint tint change on its own is not proof of much.

Storage is not fussiness. Air, light, heat and water all speed up that breakdown.

• An opaque bottle or an airless pump beats a wide-mouth jar.
• A dark drawer or cupboard beats a sunny windowsill or a hot Auckland bathroom shelf in February.
• Close it straight after use, every time.
• Buy a size you will finish in a few months. Splitting the volume across two sealed bottles, as Medik8 Pure C15 does, keeps the second one untouched while you work through the first. Medik8's own instruction is to finish one bottle before opening the second, and that is the point of the format.

On the fridge: cooler storage slowing a chemical breakdown is ordinary chemistry, but we have not found a study showing that refrigerating a serum changes how skin looks. Treat it as sensible housekeeping rather than a proven step.

Where THD ascorbate fits

THD ascorbate is the gentler-feeling option. Because it is oil-soluble, it sits in a normal-pH oil or cream base rather than a strongly acidic water base, which is why a serum like Medik8 C-Tetra feels like a light oil going on. If a low-pH serum is not comfortable on your skin, this is a sensible thing to try instead. The same chemistry is why the form works in a cream base at all, which L-ascorbic acid at pH 3.5 cannot do.

Now the honest part, because this is where the marketing gets loud and the evidence gets quiet.

The randomised trial we found ran eight weeks in 44 participants, and it was not comparing THD ascorbate against L-ascorbic acid, or against a placebo. It compared 5 per cent THD ascorbate plus 1 per cent acetyl zingerone against 5 per cent THD ascorbate on its own. In the combination group, photo analysis showed average wrinkle severity down 3.72 per cent and pigment intensity down 4.10 per cent at eight weeks, with redness down 14.25 per cent. In the THD-alone group, wrinkle severity and redness both went up instead, though the paper does not report whether those increases were statistically significant. An instrument reading of pigmentation and redness under the eye found no significant difference between the two groups (Afzal et al., 2024). Separately, laboratory work on cells found THD ascorbate degraded quickly when exposed to singlet oxygen and was a poor antioxidant on its own (Swindell et al., 2021).

Read those two with your eyes open. Both papers include an author who works for Sytheon, the company that makes the acetyl zingerone they conclude in favour of, and the trial lists Sytheon as its funder. That does not make the results wrong, but it is not independent work. The laboratory study is cells and test tubes, which cannot tell you how a face looks after eight weeks. The trial had no placebo arm, so it cannot tell you whether THD ascorbate on its own does anything at all compared with using nothing. And the improvements that were reported are a few per cent on photo analysis, which is a small change. Two of the three sat only just inside the usual threshold for calling a result statistically significant.

One more thing, since THD ascorbate is sold as the gentle one. Gentler on average is not the same as reaction-free. A 2022 report in a contact dermatitis journal describes allergic contact dermatitis linked to two eye creams containing THD ascorbate (Scheman et al., 2022). A published report cannot tell us how common that is, but it is a fair reason to patch test. Allergic contact dermatitis is a medical matter. If a reaction does not settle quickly after you stop the product, see your GP rather than working through it yourself.

Using either one sensibly

• Patch test anything new on your inner forearm for a few days before you put it on your face, and longer before you go anywhere near your eyes.
• Do not apply either form to broken, sunburnt or already irritated skin.
• Keep it out of the eye area unless the product is specifically made for it.
• Introduce one new active at a time. Layering vitamin C with a retinoid or an exfoliating acid in the same routine can leave skin feeling more sensitive, so give a new serum a few weeks on its own first.
• A brief tingle settling in a minute or two is common with a low-pH serum. Stinging that keeps going, burning, or skin that stays red is your cue to stop using it.
• Wear sunscreen every day, over the top, whichever form you use.

What these ingredients cannot do

• They do not treat any skin condition. Nothing above supports treating, curing, preventing or healing melasma, pigmentation disorders, acne, rosacea or anything else. What the studies reported is a change in the appearance and surface texture of skin over weeks to months, and the measured changes were modest.
• They are not medical care. Persistent pigmentation, melasma, rosacea, ongoing redness, a rash that will not settle, or any spot or mole that is changing are all medical matters. Please see your GP or a dermatologist. Nothing on a shelf, ours included, is a substitute for that.
• They do not replace sunscreen. None of the research here shows vitamin C gives sun protection on its own, or that it lets you use less SPF. Wear sunscreen daily whatever else is in your routine, and in New Zealand that does not change with the season. If you want fewer steps, Advanced Day Ultimate Protect is an SPF 50 day moisturiser, so it covers the moisturiser and the sunscreen in one. It is not a vitamin C product, and it goes on over your serum rather than instead of it.
• They say nothing about supplements. None of these studies looked at oral vitamin C, so none of it supports taking a tablet to change how skin looks.

So which one should you buy

Since there is no head to head human trial comparing the two at matched strengths, any claim that one is proven better goes past the evidence. Claims that THD ascorbate is a specific multiple more potent, or penetrates a set number of times better than L-ascorbic acid, come from supplier marketing rather than published clinical data. We have looked for the trial behind those numbers and have not found it.

What we would say instead is this. L-ascorbic acid has the deeper human research and the harder handling. THD ascorbate is easier to live with and much less directly proven. Both are cosmetics, both produce modest changes in how skin looks at best, and results vary from person to person.

The serum you tolerate and actually finish will do more for the look of your skin than the theoretically stronger one that stings and ends up at the back of a drawer.

If you are not sure which form suits you, come and talk to us in the Parnell store. We would rather match you to something you will use than sell you the strongest thing on the shelf.